Electrodeposition of chromium



United States Patent 3,341,434 ELECTRODEPOSITION 0F CHROMIUM Frank Passal, Detroit, Mich., assignor to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Sept. 24, 1963, Ser. No. 311,223 13 Claims. (Cl. 204--51) This invention relates to chromium plating. More particularly it relates to a novel technique for suppressing the evoluton of fumes from chromium plating baths.

As is well known to those skilled in the art, chromium plating may be effected from baths which contain chromic acid and sulfate (typically introduced as sulfuric acid) together with other ingredients or additives including those which may control or regulate the plating characteristics of thebath. Typical of such regulating additives may be strontium sulfate and potassium silicofluoride with or without additions of excess strontium ion (introduced as SrCO SrCrO etc.) or excess potassium ion (introduced as K2CIO4, K Cr O etc.) which may make the bath a high speed bath which is self-regulating with respect to catalyst anion concentrations.

During chromium plating from such baths, there is a very vigorous evolution of hydrogen from the cathode and of oxygen from the anode; a these gases escape, they carry into the atmosphere substantial amounts of entrained chromium plating solution containing chromic acid, hydrofluoric acid, sulfuric acid, etc. sufficient to produce a variety of toxic effects on personnel engaged in the operation of the bath. Cumulative effects of these fumes may cause irritation of the respiratory tract, perforation of the nasal septum or lung cancer.

The evolved gases together with the entrained chemicals therefore pose a serious problem. Particularly because they are toxic to operating personnel and because they are also corrosive to equipment or harmful to other nearby plating baths, such as that used to plate nickel, it is desired to operate in manner to minimize or elimimate the evolution of the objectionable constituents of these gases into the surrounding atmosphere. Prior art attempts to accomplish these desiderata by means of relatively inexpensive mist-suppressing organic additives have been only partially successful. Most of the prior art additives have been organic surfactants which operate by building up a head of foam on the chromium plating bath restricting the entrainment of liquids in the evolved hydrogen and oxygen. As operations continue, the foaming properties of these prior art additives quickly decrease, mainly because they may rapidly deteriorate or decompose under the severe action of the electrolyte. As a result, their value as fume suppressants may rapidly decrease. Accordingly, these prior art fume suppressants must be added to the bath in large quantities or must be frequently replenished typically after very short periods of operation. These frequent additions of rela tively large amounts of fume suppressants may result in undesirably rapid build-up of decomposition products in the bath which may undesirably affect the catalyst balance of the bath.

Those skilled in the art have attempted to develop inexpensive fume suppressants characterized by their ability to produce a durable, long-lasting blanket of foam on the surface of the plating bath. Attempts have also been made to develop fume suppressants which may produce these desired results when present in the bath at low concentrations and which need be replenished only after relatively long periods of operation.

It' is an object of this invention to provide a novel fume suppressant for electroplating baths characterized by its ability to economically generate a foam blanket of outstanding durability. It is a further object to provide novel fume suppressants which may be effective barium, magnesium, nickel, iron, and

When present in the bath at very low concentrations. Other objects will be apparent to those skilled in the art on inspection of the following description.

In accordance With certain aspects of this invention, chromium plating may be effected in a bath containing chromic acid and sulfate wherein substaintial quantities of gas may be liberated from the electrodes by the process which comprises maintaining a chromium plating bath containing chromic acid and sulfate, maintaining therein a relatively small, antimisting amount of a sulfonated 2-alkylbenzimidazole, and plating chromium onto a cathode in said bath.

In accordance with another aspect, the novel composition of this invention which is characterized by its ability to develop and maintain over an extended period of time an optimum thickness of foam, may comprise an aqueous acid chromium plating solution and, as antimisting agent, a relatively small anti-misting amount of a sulfonated 2-alkylbenzimidazole.

Typically chromium plating baths with which this invention may be employed may commonly have the following illustrative compositions:

l 1 E.g. from sulfuric acid.

2 E.g. from H SiF The novel fume-suppressant or anti-misting agent of this invention may be a compound which may typically have the structure wherein Ar is selected from the group consisting of phenyl and naphthyl rings, said ring being incorporated into the imidazole structure through vicinal carbon atoms of said ring (i.e. through two carbon atoms which are immediately adjacent to each other); M is a cation selected from the group consisting of hydrogen, ammonium and metals; R is a non-aromatic hydrocarbon radical containing at least 3 carbon atoms; R is selected from the group consisting of hydrogen, alkyl, aryl, aralkyl, omegasulfonated alkyl, sulfonated aryl, and sulfonated aralkyl; R" is selected from the group consisting of hydrogen, alkyl and aralkyl; Y is a water-soluble anion; and a is 01. The sulfonated benzimidazoles which contain alkenyl, cycloalkyl and cycloalkenyl substituents in the 2-position will be understood to be embraced by the term sulfonated 2-alkyl benzimidazole as used in the context of this invention.

In the sulfonated 2-alkylbenzimidazole, the group Ar may be phenyl or naphthyl, and preferably it may be phenyl.

The cation M may be a metal (including hydrogen and ammonium). Typically M may be selected from the group consisting of hydrogen, ammonium, sodium, potassium, lithium, trivalent chromium, calcium, strontium, copper. Preferably, M may be ammonium or an alkali metal e.g. sodium, potassium and lithium. 1

The group R may be a non-aromatic hydrocarbon containing at least 3 carbon atoms. Preferably, R may be selected from the group consisting of alkyl, cycloalkyl, alkenyl, and cycloalkenyl radicals. Preferably, R may contain' at least 8, say 818 carbon atoms. Preferred R groups may include n-octyl, isooctyl, nonyl, decyl, undecyl, tridecyl, pentadecyl, heptadecyl, octadecyl, 4- ethylcyclohexyl, octenyl, nonenyl, undecenyl, tridecenyl, pentadecenyl, heptadecenyl, 4-amylcyclohexenyl, etc.

The group R may be hydrogen, alkyl, aryl, aralkyl, omega-sulfonated alkyl, sulfonated aryl or sulfonated aralkyl. Preferably R may be omega-sulfonated alkyl, sulfonated aryl or sulfonated aralkyl and most preferably R may be selected from the group consisting of omegasulfonated lower alkyl (i.e. an alkyl containing less than about 6 carbon atoms), sulfonated phenyl and sulfonated benzyl. The sulfonate group contained in the R group may typically be of the form MO S wherein M is as defined supra and preferably wherein M is ammonium or alkali metal, e.g. sodium, potassium or lithium.

The group R may be hydrogen, alkyl or aralkyl. Preferably, R may be selected from the group consisting of hydrogen, lower alkyl (i.e. an alkyl containing less than about 6 carbon atoms) and benzyl. Most preferably, R" may be benzyl.

The group Y may be a solubilizing anion, typically a water-soluble anion. Preferably, Y may be selected from the group consisting of bromide, iodide, sulfate, acetate, methosulfate, ethosulfate, citrate, and perchlorate.

The sulfonated 2-alkylbenzimidazoles which may be present in the novel compositions of this invention may preferably contain a quaternary nitrogen atom. Thus, a may be 1 and the compound may be a quaternized sulfonated 2-alkylbenzimidazole. When a is 0, the compound as added may contain no quaternized nitrogen atom.

Examples of illustrative sulfonated 2-alkylimidazole compounds which may be used in the novel fume-suppressant compositions of this invention may include: (A) sodium 2-n-heptadecyl-3-sulfophenylbenzimidazole sulfonate (B) sodium 2-n-heptadecyl-3-sulfobenzylbenzimidazole sulfonate (C) potassium 2-n-octyl-3-omega-sulfo-n-butylbenzimidazole sulfonate (D) sodium 1-benzyl-2-n-undecyl-3-sulfobenzylsulfobenzimidazolium bromide (E) potassium 1-methyl-2-pentadecyl-3-sulfophenylf sulfobenzimidazolium acetate (F) sodium 1-ethyl-2-nonyl-3-omega-sulfo-n-propy1- sulfobenzimidazolium ethosulfate (G) ammonium 2-isooctyl-3-sulfophenylbenzimidazole sulfonate In the plating bath, the sulfonated 2-a1kyl benzimidazole is believed to exist in an ionized form such as Itwill be apparent to one skilled in the art that when R also. contains a sulfonate group, this group may also be present in the ionized 8 form. The nitrogen atoms of the imidazole ring may also be present as ionized quaternary nitrogens in the highly acidic plating bath.

The novel fume suppressants may typically be added to the bath as a solid or in aqueous solution. Typical solutions may contain about 120%, say 10%, by weight of the novel fume-suppressant compound. The dry material may also be compressed, i.e., pelletized, preferably together with a release agent, say sodium bicarbonate. Mixtures of the noted sulfonated Z-a'lkylbenzimidaz oles may also be employed.

It has been discovered that when as little as 0.001 g./l. (grams per liter) of the novel anti-misting composition is added to a chromic acid bath in the chromium plating process, the amount of mist and spray is substantially reduced. Although these additives are stable in chromic acid baths, they do decompose to a slight extent and over a period of time are used up or depleted; drag out of solution further contributes to this depletion. The minimum initial amount added to the bath is to a large extent governed by the contemplated replenishment rate. It is also in part governed by cost considerations, by the amount of plating current used and by the actual construction of the plating tanks. Baths containing this additive develop a stable Ifoam blanket on the surface. It is desirable that this blanket should be stable and of substantial, yet not too great, thickness, since too thick a foam blanket could be dangerous because of the large amount of entrapped oxygen and hydrogen, and consequent implosion hazard. For this reason the geometry of the plating system affects the desired concentration of additive as it will affect the thickness of the foam blanket for a given volume of bath. Because the novel fumesuppressants possess outstanding foaming ability, no more than about 0.2 g./l. of the additive need be present in the bath at any one time. The 'bath may preferably contain less than 0.2 g./l., say about 0.001 g./l. to 0.2 g./l. Larger amounts may be used for intermittent operation but for continuous electrolysis it is better to use only 0.2 g./l. or less, as otherwise the foam blanket may foam to an inconvenient thickness.

In typical embodiments of this invention, the novel fume-suppressant composition may be present in chromium plating baths in the amount of about 0.001 to 0.2 g./l. of bath. Preferably, the composition may be present in the amount of 0.002 to 0.05, say 0.01 g./l. of bath.

It has been found that the anti-misting properties of the novel composition of this invention may endure over an extended period of time even when a very low concentration of the fume-suppressant is used. Because of the outstanding stability and high degree of effectiveness of these fume-suppressants, contamination of the bath by decomposition products is minimized.

Practice of the invention may be observed from the following examples wherein the parts are parts by weight unless otherwise note-d.

EXAMPLE l A chromic acid platingsolution was made up to contain:

205 liters of this solution was added to a plating tank. The solution depth was 57.7 cm. and surface area was 3 570 sq. cm. Lead anodes and a steel cathode were positioned in the tank and the solution was thermostatically controlled at 45 C.

A current of amperes was passed through the tank and a vigorous evolution of spray resulted at the electrodes. This spray determined visually and by holding sheets of white paper about 5 cm. over the solution adjacent to electrodes and observing the extensive and rapid wetting and brown coloration of the under side of the paper,

To the solution there was then added 0.05 g./l. of sodium 2-n-heptadecyl-3-sulfobenzylbenzimidazole sulfo nate and thespray release was completely eliminated.

The thickness of the foam blanket was measured from time to time with the following results:

5:10 current turned otf The solution was allowed to cool to room temperature overnight.

On the following day when electrolysis was resumed spray release was effectively suppressed all day (8 hours) with a 7.6 to 15.2 cm. foam blanket at 9 am. and 2.5 to 6.4 cm. foam blanket at 5 pm.

On the third day electrolysis was resumed using 150 amperes and spray was effectively suppressed all day (8 hours). Near the end of the third day, the foam blanket began to decrease. Addition of 0.005 g./l. of the same sulfonated 2-alkylbenzi1nidazole gave substantially complete suppression of spray for an additional 8 hours.

EXAMPLE 2 Four liters of chromium plating solution containing 250 g./l. CrO and 2.5 g./l. 50.; was placed in a battery jar. The solution depth was 21 cm. and the surface area was about 195 sq. cm. The solution was heated to 49 0, two lead anodes and a brass cathode were positioned therein, and a cell current of 5 amperes Was applied. A vigorous release of spray was observed at the electrodes.

Sodium 2-n-heptadecyl-3-sulfo'benzylbenzirnidazole sulfonate was added to the solution at the rate of 0.005 g./l. (0.02 g. total). A foam blanket of about 2.5 cm. was formed and spray release was effectively suppressed during seven hours of electrolysis.

EXAMPLE 3 In a manner similar to that described in Example 2, sodium 2-n-heptadecyl-3-omega-sulfo-n-propylbenzimidazole sulfonate was tested as a spray suppressant. Addition of 0.1 g./l. of this material to a chromium plating bath gave complete spray suppression and a foam blanket of about 2.5 cm. after 6 hours of electrolysis.

From an inspection of the foregoing examples, it may readily be seen that the novel agents of this invention are characterized by their surprisingly high durability and effectiveness.

It is a particular advantage of the novel compositions of this invention that they may produce less sulfate build-up in a commercial bath than fume-suppressants previously used. Sulfate ion is produced when the fumesuppressant breaks down in the plating bath. Since the compositions of this invention may be effective for several times as long as previously used agents, they need not be added as frequently, and sulfate build-up may be thereby reduced.

It is a feature of this invention that use of the novel anti-misting agents is characterized by lower initial cost, production of more foam per unit weight of additive, longer foam life, lower concentration threshold of effectiveness, and the ability to produce durable foams over extended periods of usage.

Although this invention has been illustrated by reference to certain specific embodiments, modifications thereof which are clearly within the scope of the invention will be apparent to those skilled in the art.

What is claimed is:

1. In the process for electrodepositing chromium from a chromium plating bath, the improvement which comprises maintaining in said bath a small, anti-misting amount of a sulfonated 2-alkylbenzimidazole.

2. In the process for electrodepositing chromium from a chromium plating bath, the improvement which comprises maintaining in said bath 0.001-0.2 g./l. of a sulfonated 2-alkylbenzimidazole.

3. In the process for electroplating chromium from a chromium plating bath, the improvement which comprises maintaining in said bath about 0.0010.2 g./l. of a compound having the formula wherein Ar is selected from the group consisting of phenyl and naphthyl rings, said ring being incorporated into the imidazole structure through vicinal carbon atoms of said ring; M is a cation selected from the group consisting of hydrogen, ammonium and metals; R is a non-aromatic hydrocarbon containing 318 carbon atoms; R is selected from the group consisting of omega-sulfonated lower alkyl, sulfonated phenyl and sulfonated benzyl; R" is selected from the group consisting of hydrogen, lower alkyl and benzyl; Y is a water-soluble anion; and a is 0-1.

4. In the process for electroplating chromium from a chromium plating bath, the improvement which comprises maintaining in said bath about 0.002-0.05 g./l. of a compound having the formula R! wherein Ar is selected from the group consisting of phenyl and naphthyl rings, said ring being incorporated into the imidazole structure through vicinal carbon atoms of said ring; M is a cation selected from the group consisting of hydrogen, ammonium and metals; R is a non-aromatic hydrocarbon containing 3-18 carbon atoms; R is selected from the group consisting of omega-sulfonated lower alkyl, sulfonated phenyl and sulfonated benzyl; R" is selected from the group consisting of hydrogen, lower alkyl and benzyl; Y is a water-soluble anion; and a is 01.

5. The process for electroplating chromium as claimed in claim 4 wherein the compound is sodium 2-n-heptadecyl-3-sulfophenylbenzimidazole sulfonate.

6. The process for electroplating chromium as claimed in claim 4 wherein the compound is sodium 2-n-heptadecyl-3-sulfobenzylbenzimidazole sulfonate.

7. The process for electroplating chromium as claimed in claim 4 wherein the compound is sodium 2-n-heptadecyl-3-omega-sulfo-n-propylbenzimidazole sulfonate.

8, A novel bath for the electrodeposition of chromium comprising an aqueous acid chromium plating solution and, as an anti-misting agent, 0.00l-0.2 g./l. of a sulfonated 2-alkyl benzimidazole.

9. A novel bath for the electrodeposition of chromium comprising an aqueous acid chromium plating solution and, as an anti-misting agent, 0.0010.2 g./l. of a compound having the formula wherein Ar is selected from the group consisting of phenyl and naphthyl rings, said ring being incorporated into the imidazole structure through vicinal carbon atoms of said ring; M is a cation selected from the group consisting of hydrogen, ammonium and metals; R is a non-aromatic hydrocarbon containing 3-18 carbon atoms; R is selected from the group consisting of omega-sulfonated lower alkyl, sulfonated phenyl and sulfonated benzyl; R is selected from the group consisting of hydrogen, lower alkyl and benzyl; Y is a Water-soluble anion; and a is 0-1.

10. A novel bath for the electrodeposition of chromium comprising an aqueous acid chromium plating solution and, as an anti-misting agent, 0.002-0.05 g./l. of a compound having the formula I I R wherein Ar is selected from the group consisting of phenyl and naphthyl rings, said ring being incorporated into the imidazole structure through vicinal carbon atoms of said ring; M is a cation selected from the group consisting of hydrogen, ammonium and metals; R is a non-aromatic hydrocarbon containing 318 carbon atoms; R is selected from the group consisting of omega-sulfonated lower alkyl, sulfonated phenyl and sulfonated benzyl; R" is selected from the group consisting of hydrogen, lower alkyl and benzyl; Y is a water-soluble anion; and a is 0-1.

11. A novel bath as claimed in claim 10 wherein the compound is sodium 2-n-heptadecyl-3-sulfopheny1benzimidazole sulfonate.

12. A novel bath as claimed in claim 10 wherein the compound is sodium Z-n-heptadecyl-3-sulfobenzylbenzimidazole sulfonate.

13. A novel bath as claimed in claim 10 wherein the compound is sodium Z-n-heptadecyl 3 iomega-sulfo-npropylbenzimidazole sulfonate.

References Cited UNITED STATES PATENTS 2,036,525 4/ 1936 Graenacher 260-3092 2,053,822 9/1936 Graenacher et al. 260-3092 2,056,449 10/1936 Graenacher et al. 260-3092 2,170,474 8/ 1939 Graenacher et a1. 260-3092 X 2,846,380 8/1958 Brown 204-51 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

3. IN THE PROCESS FOR ELECTROPLATING CHROMIUM FROM A CHROMIUM PLATING BATH, THE IMPROVEMENT WHICH COMPRISES MAINTAINING IN SAID BATH ABOUT 0.001-0.2 G./L. OF A COMPOUND HAVING THE FORMULA 